Electrical machines (motors, generators) typically have, in the majority of the longitudinal grooves of their stator laminate stacks, special types of coil windings or conductor bars, generally made from copper or another material of high conductivity. In the case of an electric motor, by supplying current in a time-selective manner, a magnetic field propagating in all directions is generated, and this drives the freely rotating rotor suspended in the stator cavity, and the rotor reacts to the induced magnetic field in the form of forced rotation, for example owing to a multitude of applied permanent magnets, and hence converts electrical energy to kinetic energy. In electrical terms, the laminate stack is at ground potential, but the coils are at high kilovolt potential. The coils fitted into the stator grooves must accordingly be electrically insulated with respect to ground potential. For this purpose, each and every coil is insulated, for example, with a special tape, for example mica tape, repeatedly and with defined overlap.
Mica is commonly used since, being a particulate inorganic barrier material, especially in platelet form, it is capable of retarding electrical erosion under electrical partial discharges effectively and for a long period, for example over the entire lifetime of the machine, and has good chemical and thermal stability. Mica tapes consist of mica paper and one or more carriers, for example fabrics, film(s), bonded to one another via a tape adhesive. Mica tapes are necessary since mica paper alone does not have the mechanical strength needed for an insulation process. According to the application, additives may be added to the tape adhesive, for example curing catalysts, which have an initiating effect on the thermal curing of an externally applied impregnating agent: after the mica tape-insulated coils have been fitted into the stator laminate stacks and electrically connected, for avoidance of partial discharges during later operation, the air in the cavities of the windings and especially in the groove gaps of the stator laminate stack is eliminated. Since this distance from current-carrying insulated coil to the laminate stack is generally kept as small as possible, field strengths of several kV/mm there are not unusual. There is corresponding stress on the insulation material.
Impregnating agents according to the prior art that have been found to be suitable for vacuum impregnation processes are thermally curable epoxy resin/anhydride mixtures. They are used for impregnation of the stators of the electrical machines, composed of the individual parts thereof, with the fitted and mica tape-insulated coils, or for individual coil or conductor bar impregnation. During a VPI (vacuum pressure impregnation) process, these stators or coils are usually wholly flooded with a mobile epoxy resin/phthalic anhydride formulation in a vacuum chamber and then impregnated under pressure. The final curing is generally effected under standard pressure in an industrial oven.
The function of the curing catalyst is for the mobile impregnating agent, usually composed of epoxy resin and phthalic anhydride, to gelate within a particular period at a given temperature. The industrial standard impregnating agent in this regard has to date been a mixture of distilled bisphenol A diglycidyl ether and methylhexahydrophthalic anhydride. This mixture is sufficiently mobile to assure the complete impregnation of the tape insulation on the one hand and, in the absence of curing catalysts, sufficient storage stability on the other hand. The curing catalyst is generally at least also present in the solid insulation material, for example mica tape. This mica tape is held together by the tape adhesive, and so it is essential that the tape adhesive and the curing catalyst are inert to one another.
Typically, all three components, i.e. tape adhesive, curing catalyst, and charged impregnating agent, do not react until the moment they encounter one another during the VPI process. In this way, the best possible crosslinking and attachment, compatibility and freedom of the insulation from cavities are achieved, which leads in turn to an optimized lifetime of the “main insulation” of the electrical machine formed thereafter in the course of curing. Owing to toxicological concerns about the unrestricted use of phthalic anhydrides, impregnating agents used in the future will be phthalic anhydride-free or completely anhydride-free epoxy-based impregnating agents, which are polymerized using curing catalysts.
The novel curing catalysts are matched to the anhydride-free impregnating agents. There is increasing use of anhydride-free impregnating agents, as known from the prior applications DE 102015214872.6 and DE 102015213534.9, the disclosure content of which is hereby incorporated into the present description. These propose the use on the one hand of imidazoles and/or pyrazoles and the derivatives thereof as curing catalysts, and on the other hand covalently bridged diimidazole derivatives and/or covalently bridged dipyrazole derivatives as curing catalysts which, for example, are condensation products and/or addition products. These are curing catalysts in solid insulating materials which, by virtue of the molecular enlargement and possible additional interactions at the formerly electrophilic center, have a lower volatility than the simple (alkyl)imidazoles. In spite of this lower volatility, the reactivity with respect to acid anhydride-free impregnating resins based on epoxy resin is adversely affected only insubstantially, or not at all, in comparison to simple (alkyl)imidazoles. Consequently, these systems represent excellent curing catalysts for acid anhydride-free impregnating resins based on epoxy resin.